Generally an XSLT transform transforms a document from one nature into another.
We may use a convention that the purpose of an XSLT transform is the nature of what results.
The nature of what is to be transformed can be described by the URI reference from which the RDDL document is obtained (i.e. the namespace URI of what is being transformed).
By employing this convention we can identify a transform of "A" -> "B" by looking in the RDDL document of the namespace "A" for a resource having a purpose of the namespace "B".
(The nature of this resource will be http://www.w3.org/1999/XSL/Transform indicating that it is an XSLT transform.)
An XSLT extension is known by the purpose URI http://www.rddl.org/purposes/software#xslt-extension.